/*
 * Copyright 2002-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.beans.factory.serviceloader;

import java.util.ServiceLoader;

import org.springframework.beans.factory.BeanClassLoaderAware;
import org.springframework.beans.factory.config.AbstractFactoryBean;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;

/**
 * Abstract base class for FactoryBeans operating on the
 * JDK 1.6 {@link java.util.ServiceLoader} facility.
 *
 * @author Juergen Hoeller
 * @see java.util.ServiceLoader
 * @since 2.5
 */
public abstract class AbstractServiceLoaderBasedFactoryBean extends AbstractFactoryBean<Object>
        implements BeanClassLoaderAware {

    @Nullable
    private Class<?> serviceType;

    @Nullable
    private ClassLoader beanClassLoader = ClassUtils.getDefaultClassLoader();


    /**
     * Specify the desired service type (typically the service's public API).
     */
    public void setServiceType(@Nullable Class<?> serviceType) {
        this.serviceType = serviceType;
    }

    /**
     * Return the desired service type.
     */
    @Nullable
    public Class<?> getServiceType() {
        return this.serviceType;
    }

    @Override
    public void setBeanClassLoader(@Nullable ClassLoader beanClassLoader) {
        this.beanClassLoader = beanClassLoader;
    }


    /**
     * Delegates to {@link #getObjectToExpose(java.util.ServiceLoader)}.
     *
     * @return the object to expose
     */
    @Override
    protected Object createInstance() {
        Assert.notNull(getServiceType(), "Property 'serviceType' is required");
        return getObjectToExpose(ServiceLoader.load(getServiceType(), this.beanClassLoader));
    }

    /**
     * Determine the actual object to expose for the given ServiceLoader.
     * <p>Left to concrete subclasses.
     *
     * @param serviceLoader the ServiceLoader for the configured service class
     * @return the object to expose
     */
    protected abstract Object getObjectToExpose(ServiceLoader<?> serviceLoader);

}
